This invention relates to a new apparatus and method of mapping acoustic pressure fields. It utilizes the interaction of the acoustic field with a high frequency electromagnetic wave in a distributed transducer. The inherent capability of the concept of operating with high date rates without undue complexity of implementation renders it attractive to applications requiring high resolution and high speed.
The concept has the potential of giving rise to novel technologies in the areas of acoustic imaging, acoustic holography, and sonar. The apparatus of this invention has applications for mapping acoustic fields in the lower acoustic to radio-frequency ranges up to about 10MHz. It may be used in the air, in water, or underground for seismic applications.
Present methods of mapping acoustic fields are either based on scanning the field with a single directional transducer, or sampling it with a number of individual transducer elements combined in an array. The former exhibits an inherent slowness as a result of bandwidth limitations, while the latter requires parallel access to all individual transducer elements of the array and hence a multitude of electrical connections. Since for a given frequency the resolution of the array will depend on the number of its transducer elements, as well as on its spatial dimensions, a limit on resolution is unavoidably imposed by considerations of practicality. In addition, transducer arrays with individual discrete elements possess unfavorable characteristics if exposed to a nonstationary fluid medium. The periodicity of the arrangement of the individual transducer elements within the array is directly related to a strong response to flow-induced noise. This tends to impede considerably the performance of the transducer array when operating on a moving platform, such as surface ships or submarines.
A key advantage of the invention herein disclosed is reflected in the fact that only one pair of terminals are required, rather than one pair for each transducer element of a conventional array. This is a direct consequence of the manner in which the acoustic waves interact with electromagnetic waves in a new electrostatic transducer configuration. This new concept, the feasibility of which rests on the high ratio of the propagation speeds of electromagnetic and acoustic waves (the latter can be considered stationary in one sampling period), also offers significant advantages in the area of beamforming. The continuous sensitivity and relative smoothness of the transducer surface provides a high degree of flow-noise rejection if operated in a nonstationary fluid medium.